This invention relates generally to devices for shifting the phase of microwave energy propagating in waveguides, and more particularly to such devices applicable to microwave cooking appliances.
Non-uniform spatial energy distribution of microwave energy in the cooking cavity of microwave ovens is a problem of long standing for such appliances.
One approach to this problem has been to employ phase shifting devices in the feed waveguides. One example of such an approach is described in commonly-assigned U.S. Pat. No. 4,301,347 in which a phase shifter is used in combination with a circular polarizing element to radiate microwave energy into the cooking cavity with rotating elliptical polarization. The phase shifter described therein is a mechanical phase shifter comprising a resonant loop secured to a shaft journalled in the narrow walls of the waveguide, which shaft is rotated by magnetron cooling air. Reference is also made to the use of conventional electronic phase shifters, either solid state or ferrite.
In commonly-assigned, copending U.S. patent application, Ser. No. 411,153, filed Aug. 25, 1982 by Bakanowski et al, an array of slots is arranged along the waveguide to support a substantially stationary first radiating pattern when a first phase relationship for the standing wave exists in the waveguide, and a second radiating pattern when a second phase relationship exists in the waveguide. Phase shifting means is employed to periodically change the phase relationship to switch radiating patterns in the cooking cavity. The mechanical phase shifting means used in Bakanowski et al includes a solenoid actuated plunger positioned a quarter wavelength from the waveguide termination, which is inserted into the waveguide to physically shift the short circuit termination from the end wall to the plunger position, and a rotatable planar conductive vane which when oriented parallel to the broad walls has a minimal effect on the phase of the standing wave, but when oriented transverse to the broad walls provides a short circuit termination.
Mechanical phase shifters, such as employed in the Bakanowski system, provide the desired phase shift but include some less desirable features. The physical movement of metallic probes or vanes involve metal touching or closely approaching the waveguide walls, presenting the possibility of current arcing and contact wear.
The use of longitudinally movable metallic termination devices which in effect move the conductive end wall of the waveguide can be used to selectively vary the phase shift of the standing wave in the waveguide. However, arcing problems are severe at the interface of the waveguide side walls, particularly in typical microwave oven waveguide configurations where the height-to-width ratio for the guide is small, resulting in a relatively high voltage gradient per unit height. In addition, the metal-on-metal movement must overcome a relatively high coefficient of friction and is subject to considerable wear. Finally, the amount of shift introduced is equal to the longitudinal displacement of the metallic termination device; thus, to introduce a quarter wave phase shift, the device must move a distance equal to a quarter guide wavelength. Frequently, such displacement requires complex moving means and may require more space to accommodate the means for moving the device than would be preferred.
The insertion of dielectric material into a waveguide to change the phase of the standing wave in the guide is a well known technique. However, the phase shift in regions of the waveguide relatively remote from the material depends upon the presence or absence of the material in the guide, but not the relative longitudinal position of the material in the waveguide. Thus, longitudinal movement of a dielectric slab in a typical waveguide will not appreciably change the phase of the standing wave in regions of the guide relatively remote from the slab. Thus, while a dielectric slab avoids the arcing problem and the friction and mechanical wear problems inherent with movable conductive metal parts, use of dielectric slabs in conventional fashion does not provide the capability to selectively vary the phase of the standing wave throughout the waveguide by movement of the slabs in the waveguide.
In view of the advantageous use of phase shifters in relieving the problems of non-uniform energy distribution in the microwave cooking appliances and in view of the drawbacks of known mechanical phase shifting devices for such purposes, a mechanical phase shifter comprising a low-cost, non-metallic moving part to selectively shift the phase of the standing wave in the waveguide would be highly desirable.
It is therefore an object of the present invention to provide a phase shifting device applicable to microwave cooking appliances which incorporates low-cost, non-conductive moving parts which provide the desired phase shift, while substantially reducing the occurrence of high current arcing and high voltage breakdown in the waveguide of the appliance.